In recent years there has been active research in the area of nanosecond-type pulse generation. Such research has produced devices that utilize a high power photoconductive solid state switch coupled to a storage device. In order for such a device to produce a nanosecond-type pulse, the photoconductive switch must have the ability to transition from a high resistivity state to a conductive state in a sub-nanosecond time interval. One such switch, disclosed in U.S. Pat. No. 5,028,971, issued to Anderson H. Kim et al on Jul. 2, 1991, entitled, "High Power Photoconductor Bulk GaAs Switch" is incorporated herein by reference.
This GaAs switch is comprised of two mutually opposite gridded electrodes separated by a GaAs substrate capable of electrical energy storage. The stored energy can be photoconductively discharged when it receives laser light. More specifically, when the laser light is applied to the switch electron-hole pairs are generated in the substrate, thus causing the electrical resistance of the semiconductor material to instantaneously decrease. This resistance change causes the stored energy to instantaneously discharge current through an output circuit. Such instantaneous discharge of current causes an RF pulse to radiate in a direction perpendicular to the substrate.
It is widely recognized that the bandwidth of such RF radiators increases as the width of the radiated RF pulse narrows. Consequently, it has become very desirable for those skilled in the art to construct devices capable of generating faster rise time pulses.
The critical element in generating such fast rise time pulse is the properties of the energy storage device. Heretofore, there are two general techniques used to generate fast rise time, high power pulses. The first technique utilizes the recombination property of the semiconductor material from which the switch itself is fabricated. Pulses generated with this technique, however, typically have a long recovery time at high bias voltage. This long recovery time has been attributed to the substantially long recombination time and the switch lock-on phenomena exhibited by gallium arsenide. A device having such characteristics is not desirable for the many applications that require high power, ultra-wideband pulses.
The second technique utilizes an energy storage element which is comprised of either a short section of transmission line or a capacitor. The energy storage element is photoconductively triggered to instantaneously discharge all or substantially most of its stored energy to an impedance load. As with the aforementioned technique, the extended recovery time inherent in photoconductive switches prevents this device from producing extended wideband radiation.
A major breakthrough in this pulsewidth problem, however, was solved in the inventors copending patent application entitled "Ultra-Wideband High Power Photon Triggered Frequency Independent Radiator," Ser. No. 07/946,718, filed by Kim et al, Sep. 18, 1992 and incorporated herein by reference. This frequency radiator combines an energy storage function and an antenna radiation function into one structure to create an ultra-wideband frequency radiator capable of generating RF pulses with a range of frequency components from hundreds of megahertz to several gigahertz. Basically, this radiator utilizes two identical quasi-radial transmission line structures to store electric energy while it implements photoconductive switching to trigger the instantaneous discharge of the stored energy to generate the desired ultrawideband RF radiation.
Such an energy storage device comprises a dielectric storage medium, two quasi-radially shaped, metalized electrodes mounted opposite one another on the top surface of the dielectric storage medium and a metalized electrode mounted on the bottom surface of the dielectric medium. A photoconductive switch, centrally located on the dielectric between the two quasi-radially shaped electrodes, connects the two quasi-radially shaped electrodes to the bottom electrodes through a load impedance. When the switch is activated by light radiation, the stored energy discharges through the load impedance generating a sub-nanosecond type pulse.
It has been recognized by those skilled in the art that the shape of the electrodes directly affects the radiation bandwidth of the generator, because the shape directly affects the width of the discharged pulse. Specifically, the shape of the electrode directly affects the charging characteristics, and thus the discharging characteristics of the stored energy.
It has also been recognized that the distance (gap) between electrodes directly affects the energy storage capability. The larger the gap between the electrodes, the more energy the device can store before surface flashover and thus device breakdown can occur. If the gap between the electrodes is too wide, however, the bandwidth of the radiated pulse, upon discharge, will be narrowed.
Consequently, those skilled in the art recognize the tradeoff between a radiator having too narrow a gap (which provides for a wider radiation bandwidth, but a greater chance of surface flashover), and a radiator having too wide a gap (which provides a narrower radiation bandwidth, but .lesser chance of surface flashover). As such, the need for a GaAs radiator that can provide a higher power pulse without degrading the radiation bandwidth has been recognized.